The present invention relates to a dip surface-treatment system, particularly to an electrodeposition coating system, and a method for treating the surface of an article (work) with a liquid material by dipping, using the system.
In general, a car body is formed thereon with a primer coating layer, a surface coating layer, and a top coating layer. The primer coating layer is usually formed by electrodeposition coating wherein a liquid coating material (primer) is applied to a car body by dip coating. Dip coating is also used in the degreasing and chemical conversion coating on the car body, prior to the formation of the primer coating layer. In such dip coating, car bodies, which are continuously transported to the dip coating site, are fully dipped in turn in a liquid material of a tank for a certain period of time. In electrodeposition coating, it is necessary to stir or circulate a liquid coating material continuously or intermittently in order to prevent the precipitation of a pigment of the liquid coating material. Once the pigment precipitates in the tank, it is very difficult to fully disperse the pigment in the liquid coating material due to a large quantity of the liquid coating material in the tank. If the pigment dispersion in the liquid coating material of electrodeposition coating is not uniform, the gloss of the primer coating may deviate from that as originally designed. This may cause an adverse effect on the top coating. In electrodeposition coating, when an article is dipped in a liquid coating material, the paint particles are attracted to the article and deposit on its surface. Upon this, hydrogen or oxygen bubbles are generated from the surface of the article. These bubbles may cause defects of the primer coating layer. Thus, it is necessary to stir or circulate the liquid coating material to remove the bubbles from the surface of the article, too. In electrodeposition coating, heat of reaction is generated when the primer coating layer is formed on an article. With this, the liquid coating material in the vicinity of the surface of the article will increase in temperature. This will lower the resistance of the primer coating layer. If the liquid coating material is allowed to stand still under this condition, the primer coating layer may become locally too thick in thickness. In order to prevent this problem, it is also necessary to stir or circulate the liquid coating material, thereby to cool down the liquid coating material of higher temperature by supplying that of lower temperature.
Prior to the pretreatment of electrodeposition coating, a car body is formed by welding panels together, and then washed several times in order to remove metal powder and other contaminants in the welding step. It may be difficult, however, to completely remove contaminants from the car body, prior to the step of electrodeposition coating. Once contaminants (e.g., metal powder) are brought into the tank of electrodeposition coating, the contaminants may deposit on the primer coating layer. Thus, it is also necessary to stir or circulate the liquid coating material in order to remove the contaminants therefrom. In fact, the liquid coating material is filtered to remove the contaminants. The stirring or circulation of the liquid coating material may be conducted to have a flow rate, for example, of about 10 cm/s. Each of Japanese Patent First Publications 6-272091, 6-272092, 6-280095 and 8-41687 discloses an electrodeposition coating system. This system has a major tank for receiving therein a liquid coating material. The major tank has an introductory region into which an article (work) is introduced, and an exit region from which the article is withdrawn. The system further has a minor tank adjacent to the exit region of the major tank. The minor tank continuously receives an overflow of the coating liquid, and then the coating liquid in the minor tank is continuously returned to the major tank. The liquid coating material of the major tank is circulated to have (1) a surface layer's flow in a direction from the introductory region towards the exit region of the major tank and (2) a bottom layer's flow in a direction from the exit region towards the introductory region. In other words, the direction of the surface layer's flow is opposite to that of the bottom layer's flow, and these flows form a so-called loop flow circulating in the major tank. It should be noted that the article passes through the major tank in a direction along the direction of the surface layer's flow of the liquid coating material.